Marine green alga such as Ulva lactuca and Monostroma nitidum are known to form a thallus or become single cells as they become purified when grown in defined synthetic media in indoor culture (L. Provasoli; Ulva. Biol. Bull., 1958, 114, 375. M. Tatewaki, L. Provasoli and I. J. Pintner; J. Phycol., 1983, 19, 409). Moreover, it is also known that when some soil extracts, red alga extracts, brown alga extracts, or certain extracts from marine-derived microorganisms are added to the alga that lost their morphologies in this way, the thallus is formed and the propagation speed increases. (M. Tatewaki, L. Provasoli and I. J. Pintner; J. Phycol., 1983, 19, 409.) However, their active elements and vitamin-like active substances have not been identified, which creates a difficulty in studying their physiology and life or to maintain the culture for a long period using the foliate green alga such as Ulva lactuca, and Monostroma nitidum under aseptic conditions such as indoor cultivation.